1. Field of the Invention
The present invention relates to a development apparatus which develops an electrostatic image formed in an image bearing member with a developer, a copying machine having the same, and an image forming apparatus such as a laser beam printer and a facsimile.
2. Description of the Related Art
With reference to electrophotographic apparatuses, a development apparatus with a magnetic brush developing system which uses a two-component developer containing toner particles and a magnetic carrier has been widely used.
The developing process will be described with reference to FIG. 9. In FIG. 9, a magnet roller 45 is fixedly disposed in a developing sleeve 40, agitating screws 41 and 42 can agitate, a control blade 43 is placed on the surface of the developing sleeve in order to form a thin layer of a developer, and a developing container 44 is a container.
The agitating screws 41 and 42 convey the developer in the opposite direction each other and the developer is passed from one agitating screw to the other at both ends. As a whole, it is conveyed so as to be circulated in one direction.
The developer pumped by a N2 pole in accordance with the rotation of the developing sleeve 40 is regulated by a control blade 43 in the process of conveyance from a S2 pole to a N1 pole and then a thin layer is formed on the developing sleeve 40. When the developer in which a thin layer is formed is conveyed to a S1 pole which is a developing main pole at the time, a spike-like shape of the developer is formed by a magnetic force. The electrostatic image is developed by the spike-like shape of the developer and then the developer on the developing sleeve 40 is returned to a developing container 44 by the repulsive magnetic field of a N3 pole and the N2 pole. With reference to the two-component development, generally, homopolar magnetic poles are arranged and disposed as described above and the developer after the development is once released from the developing sleeve not to leave an image history.
In recent years, the miniaturization, high-quality image, and long-life technologies for the development apparatus and the image forming apparatus using a two-component development device have been developed. In order to ensure the long life of development apparatus, it is necessary to reduce the load on the developer and prevent the degradation of toner and carrier.
The location in which the load is applied to the developer in the developing container is a developer layer thickness regulating portion. In a usual structure, the developer layer thickness regulation pole is located upstream of rotational direction of the developing sleeve 40 rather than the control blade 43. The developer drawn to the developer layer thickness regulation pole in the region is compressed between the developing sleeve 40 and the container.
Specific methods which reduce the load on the developer in the developing sleeve 40 in order to ensure a long life of the developer are disclosed in Japanese Patent Application Laid-Open Nos. 11-194617 and 2000-206792. These methods involve the steps of reducing the magnetic field strength in a direction perpendicular to the surface of the developing sleeve 40, decreasing the amount of magnetization of carrier, using one of the repulsive magnetic poles as the developer layer thickness regulation pole, and reducing the amount of the developer drawn to the developer layer thickness regulation pole.
However, the screw-pitch shaped density unevenness may be generated in a black-colored image in the case where the compression of the developer in the developing sleeve 40 is weakened.
The phenomenon is caused by the result that when the developer is supplied to the developer layer thickness regulation pole by rotation of the screw 41 in the vicinity of the developing sleeve 40 with unevenness in the toner electrification amount Q/M, the developer is conveyed to the developing portion as it is. Such a phenomenon is easily generated, particularly when a degree of the compression of the developer in the developer layer thickness regulation pole is reduced.